Spectrum analyzers for determining the frequency spectrum of an input signal which is stationary in time have been available for several years. These frequency analyzers rely, however, upon the stationary nature of the input signal. Accordingly, they are unable to determine the frequency spectrum of or pulsed signals.
It is, however, often desirable to obtain the spectral components of a signal which has a pulsed nature. As used herein pulsed signals refer to signals which are not stationary in time wherein the input signal is characterized as having a plurality of active periods separated by inactive periods. Typically the pulsed signal is stationary during the active period. Devices which use pulsed signals include video cassette recorders (VCRs), disk drives, facsimile machines and multiplexers.
Prior art spectrum analyzers are incapable of measuring the frequency spectrum of pulsed signals due to the noncontinuous nature of the frequency of the pulsed signals. If an attempt is made to measure the active period of a pulsed signal with a prior art spectrum analyzer, the analyzer will detect the spectral components of both the active and inactive portions of the pulsed signal along with frequency components corresponding to the repetition rate of the pulsed signal. Measuring pulsed signals in this manner has several problems.
As an example, noise components or signals of interest may be buried under a frequency component present in the undesired inactive portion of the pulsed signal. In similar manner, noise components may be buried under one of the frequency components associated with the pulse repetition rate of the pulsed signal. If both the active and inactive portions of the pulsed signal contain a common frequency component, it is not possible to separate the contribution of this component to the frequency spectrum that is caused by the active portion of the pulsed signal from the contribution caused by the inactive portion of the pulsed signal. Analysis of the results can be confusing because of the presence of the frequency components associated with the pulse repetition rate. Analysis is further frustrated because the frequency components associated with the active and inactive portions of the pulsed signal appear to come and go at a time rate which is dependent on the sweep rate of the spectrum analyzer and the pulse repetition rate of the pulsed signal.
Others have attempted to modify prior art analyzers thereby to make them suitable for measuring pulsed signals. Such modifications have been implemented by sweeping the local oscillator of the spectrum analyzer only during the active portion of the signal using a trigger signal supplied by a sweep signal generation circuit external to the spectrum analyzer. Using such a modification and collecting and displaying data only during the time the local oscillator was sweeping would allow the spectrum of the pulsed signal to be analyzed without the effects of the frequency components produced by the inactive portions of the pulsed signal or the pulse repetition rate.
Such modifications to prior art spectrum analyzers have several disadvantages. One disadvantage is the need to generate timing signals in the sweep signal generation circuit external to the spectrum analyzer. A sweep signal generation circuit external to the analyzer must be used to generate a trigger signal that rises a first predetermined period of time after the rising edge of the active portion of the pulsed signal and falls a second predetermined period of time before the trailing edge of the active portion of the pulsed signal. Additionally, since the first predetermined time period is a function of the selected analyzer bandwidth, a means of selecting variable time would need to be provided if the analyzer were to be used with multiple bandwidth selections. Experience has shown that the timing signals are difficult to provide with the necessary accuracy.
A related disadvantage results from the desire to obtain as many conversions, i.e., data samples, as possible during the active portion of the pulsed signal. Such circuitry requires the trigger signal provided by the sweep signal generation circuit to anticipate, or predict, exactly when the active period of the pulsed signal will end so that no conversions are taken after the active period, i.e., during the inactive period. Failure to discontinue the trigger signal prior to the end of the active period of the pulsed signal will result in conversions of the pulsed signal being taken during the inactive period and corruption of the measurement, as discussed above.
A third disadvantage is the presence of spectral amplitude errors which occur during the portion of the pulsed signal that is frequency converted near the beginning and end of the measurement period. Using the above described modification to the prior art spectrum analyzers, the starting and stopping of the frequency sweep of the local oscillator creates a measurement period during which the pulsed signal is frequency converted and data is collected. Due to the nature of real versus ideal circuit components, amplitude errors are introduced in the frequency conversion at the beginning and end of the measurement interval. Such amplitude errors result from circuit propagation delays and oscillator transients when the data collection and sweeping of the local oscillator are performed synchronously. No known modification to prior art spectrum analyzers exists for compensating for such errors.
It is, therefore, desirable to provide a method and apparatus for measuring the frequency spectrum of a pulsed signal wherein the measurement is not affected by inactive portions of the pulsed signal. It is also desirable to provide a method and apparatus for measuring the spectral components of a pulsed signal wherein the measurement is not affected by frequency components due to the pulse repetition rate of the pulsed signal. It is further desirable to provide a method and apparatus for measuring the frequency components of a pulsed signal wherein the measuring apparatus and method is capable for compensating the measurement data for erroneous measurements taken during inactive portions of the pulsed signal. It is still further desirable to provide a method and apparatus for measuring the frequency spectrum of a pulsed signal wherein the measurement is not contaminated by amplitude errors occurring during the measurement.